doc + micro optimization

Optimizations:
- one cmp less in loops (-> do while)
- optimized loops for branch prediction
remove grow case differentiation

benchmarks:
http://play.golang.org/p/xMWTHiu9JY

https://groups.google.com/forum/?fromgroups=#!topic/golang-nuts/ETbw1ECDgRs
fix build
fix build with Go 1.0 + branch optimization

inverted loop condition, so that in common case it matches the previous
branching result
Merge pull request #90 from go-sql-driver/buffer

Buffer optimizations